Light emission from silicon has been a topic of intense research interest due to its potential use in displays, general illumination, and the integration of optoelectronics with silicon technology (Lockwood, D. J., Ed., Light Emission in Silicon From Physics to Devices, New York:Academic Press (1998)). Most of this research has been carried out on porous silicon (PSi). However, it is now well understood that visible photoluminescence (PL) from PSi arises from silicon nanocrystals within it (Nirmal et al., Acc. Chem. Res. 32:407 (1999)). For many applications, well-characterized discrete nanocrystals would be much more useful than PSi. In silicon nanocrystals less than about 5 nm in diameter, quantum confinement widens the band gap, leading to particle size -dependence of the PL wavelength, from blue to the near infrared. Finite size effects also increase the probability of radiative recombination of excitons and decrease the possibility of non-radiative recombination (Nirmal et al., Acc. Chem. Res. 32:407 (1999)). Both non-radiative and radiative recombination may occur at the nanocrystal surface as well as within the nanocrystal. In particular, oxygen at the surface may introduce states within the bandgap that lead to loss of emission or red-shifted emission (Wolkin et al., Appl. Phys. Lett. 82:197 (1999)).
Several methods have been developed for preparing luminescent silicon nanoparticles outside of porous silicon, including fracturing of porous silicon by ultrasonication (Belomoin et al., App. Phys. Lett. 80:841 (2002), Yamani et al., J. Appl. Phys. 83:3929 (1998)), inverse micellar growth (Wilcoxon et al., Phys. Rev. B. 60:2704 (1999)), laser ablation (Carlisle et al., Chem. Phys. Lett. 326:335 (2000), Carlisle et al., J. Electron Spectroscopy 229:114-116 (2001)), thermal decomposition of organosilane precursors in supercritical solvents (English et al., Nano Letters 2:681 (2002), Holmes et al., J. Am. Chem. Soc. 123:3743 (2001)), solution phase methods (Baldwin et al., J. Am. Chem. Soc. 124:1150 (2002), Baldwin et al., Chem. Commun. 17:1822 (2002), Bley et al., J. Am. Chem. Soc. 118:12461 (1996), Heath, J. R. Science 258:1131 (1992)), plasma decomposition of silane (Bapat et al., J. App. Phys. 94:1969 (2003), Shen et al., J. Appl. Phys. 94:2277 (2003)), and vapor phase thermal decomposition of silane or disilane (Littau et al., J. Phys. Chem. 97:1224 (1993), Schuppler et al., Phys. Rev. Lett. 72:2648 (1994), Wilson et al., Science 262:1242 (1993)). However, two major barriers have prevented the further investigation and application of free silicon nanoparticles: (i) lack of stability of the PL properties and surface state of the particles, and (ii) difficulty in producing macroscopic quantities of high-quality particles as stable colloidal dispersions.
A method for preparing relatively large quantities of brightly luminescent silicon nanoparticles was previously developed, in which non-luminescent, crystalline nanoparticles (˜5 nm diameter) are prepared by CO2 laser heating of SiH4-H2-He mixtures, and are then etched with a HF/HNO3/water mixture to reduce their size and passivate their surface (Li et al., Langmuir 19:8490 (2003)). By controlling the etching time and conditions, the particle luminescence could be tuned from green to the near-IR. However, the green and yellow emitting particles produced by this method are unstable in air. On time scales of minutes to hours, their emission red shifts and/or decreases in intensity due to surface oxidation. Formation of an organic monolayer by hydrosilylation is an effective means of stabilizing the surface of PSi (Buriak, J. M., Chem. Rev. 102:1271 (2002), Waltenburg et al., J. Chem. Rev. 95:1589 (1995)). Several groups have reported grafting of 1-alkenes or alkynes onto H-passivated PSi or silicon wafers (Linford et al., J. Am. Chem. Soc. 115:12631 (1993), Linford et al., J. Am. Chem. Soc. 117:3145 (1995), Boukherroub et al., J. Am. Chem. Soc. 121:11513 (1999), Buriak et al., J. Am. Chem. Soc. 121:11491 (1999), Buriak et al., J. Luminescence 80:29 (1999), Buriak et al., J. Am. Chem. Soc. 120:1339 (1998)). This technique has been extended to discrete nanoparticles, to produce clear, stable dispersions of them in polar or nonpolar solvents and to stabilize the PL properties of orange-emitting particles (Li et al., Nano Letters 4:1463 (2004), Ruckenstein et al., Adv. Coll. Inter. Sci. 113:43 (2005), Li et al., Langmuir 20:4720 (2004)).
Si particles ranging from 1 to 5 nm in diameter exhibit PL emission ranging from blue to the near infrared. The small size of these nanocrystals widens the band gap due to quantum confinement effects, relaxes the selection rules that decrease the efficiency of radiative recombination in indirect band gap materials, and reduces the probability of nonradiative recombination at defects. Both non-radiative and radiative recombination may occur at the nanocrystal surface, as well as within the core of the nanoparticle. Therefore, the origin and characteristics of the luminescence are associated with both the crystallite size and with the nature of the crystallite surface. In particular, as Wolkin et al. pointed out, the presence of oxygen at the particle surface can introduce states within the bandgap that lead to red-shifted emission (Wolkin et al., Phys. Rev. Lett. 82:197-200 (1999)). An effective means of stabilizing the silicon surface and photoluminescence properties is to graft an organic monolayer to it through hydrosilylation of a hydrogen-terminated surface (Li et al., Nano Letters 4:1463-1467 (2004), Li et al., Langmuir 20:47204727 (2004), Buriak, J. M., Chem. Rev. 102:1271-1308 (2002), Buriak et al., J. Luminescence 80:29-35 (1999)) or through organosilane chemistry on a hydroxyl-terminated surface (Li et al., Langmuir 20:47204727 (2004), Ruckenstein et al., Adv. Coll. Inter. Sci. 113 :43-63 (2005), Li et al., Langmuir 20:1963-1971 (2004)). However, this has not generally been possible for blue-emitting silicon nanoparticles (˜1 nm in diameter). Preparation of organically-protected silicon nanoparticles with blue emission that remains stable in air has been a persistent challenge that has limited the possibility of using silicon nanoparticles in potential applications such as full-color displays or general illumination where full coverage of the visible spectrum is required. Alkyl-capped silicon nanoparticles prepared in solution have shown UV to blue photoluminescence, but in that case the alkyl groups were readily displaced from the surface by reaction with air (Yang et al., J. Am. Chem. Soc. 121:5191-5195 (1999)). More recently, siloxane-coated silicon nanoparticles with blue emission were prepared in solution and shown to be very stable against further reaction or degradation (Zou et al., Nano Letters 4:1181-1186 (2004). Silicon nanoparticles with blue emission have also been prepared from porous silicon (Belomoin et al., Appl. Phys. Lett. 80:841-843 (2002)).
The present invention is directed to overcoming these and other deficiencies in the art.